fix(update): add UpdateFrequency::Never to opt out of update checks

[tmchow]

Summary

Add UpdateFrequency::Never so users can opt out of forge's auto-update version check. Today the [update] frequency enum has only daily, weekly, and always, and auto_update: false doesn't help because the version check itself blocks before any prompt fires.

Why this matters

The reporter on #3119 explains the use case: low-internet / OTG users want to bring up forge to check configuration or chat history without waiting on the GitHub-side update check. Setting auto_update: false only suppresses the install prompt - the HTTP round-trip to update_informer still runs unconditionally inside on_update, so users in poor-network conditions still pay the wall-clock cost.

There was no escape value before this PR: frequency = "always" means "check every time", daily and weekly still hit the network on the first invocation per window, and there was no never.

Closes #3119. (issue: #3119)

Testing

• cargo test -p forge_domain -p forge_config -p forge_main -> 951 pass, 13 ignored. Includes a new round-trip test in compact.rs for [update] frequency = "never" and a new test in update.rs that asserts the early-return guard short-circuits before the informer.
• cargo clippy -p forge_domain -p forge_config -p forge_main --all-features --all-targets -- -D warnings -> clean
• cargo clippy --all-features --workspace -- -D clippy::string_slice -D clippy::indexing_slicing -D clippy::disallowed_methods -> clean

This contribution was developed with AI assistance.